Field of the Invention
The present invention relates to a cyclic carbodiimide compound, a polyester film using the cyclic carbodiimide compound, a back sheet for a solar cell module, and a solar cell module.
Background Art
A solar cell module generally has a laminate structure in which a transparent filling material (which is hereinafter also referred to as a sealing material)/a photovoltaic cell/a sealing material/a back sheet (which is hereinafter also referred to as BS) are laminated in this order on a glass or front sheet on the light-receiving surface side to which sunlight is incident. Specifically, the photovoltaic cell is generally embedded with a resin (sealing material) such as EVA (an ethylene-vinyl acetate copolymer) and the like, and protective sheet for a solar cell is adhered thereto. Further, as this protective sheet for a solar cell, a polyester film, in particular, a polyethylene terephthalate (which is hereinafter also referred to as PET) film has been used.
However, the protective sheet for a solar cell, above all, a back sheet (BS) for a solar cell module, which becomes in particular an outermost layer, is considered to be under an environment exposed to weather outdoor for a long period of time, and therefore, excellent weather resistance is required.
Here, a polyester film such as PET and the like, which is also used as a back sheet for a solar cell module has excellent heat resistance, mechanical characteristics, chemical resistance, and the like, and therefore, is widely used industrially. However, it still needs to be improved. As the technology for improving the characteristics of the polyester film in this manner, it is known that by reacting a carbodiimide compound with a terminal carboxyl group of polyester, hydrolysis resistance is improved, and applications to a PET film for a solar cell back sheet or the like have been studied. However, since a general carbodiimide compound generates isocyanate having a low molecular weight by reaction with a terminal carboxyl group of polyester, irritant gases volatilize in the production step, and thus, it has become a problem.
In recent years, a cyclic carbodiimide compound is proposed, and it is reported that when producing a film in a melt state by adding polyester, it is possible to suppress isocyanate gas (refer to Patent Literatures 1 and 2). In Patent Literatures 1 and 2, it is described that since isocyanate which is generated by a reaction with a terminal carboxyl group of polyester is linked to a terminal of polyester, the cyclic carbodiimide can suppress volatilization of isocyanate gas.